In the semiconductor device having the semiconductor switching element, an increase in channel density is effective in allowing a larger current to flow. In a silicon transistor, in order to increase the channel density, a MOSFET having a trench gate structure is employed, and put into practical use. The trench gate structure can be also applied to an SiC semiconductor device. However, when the trench gate structure is applied to SiC, because SiC is 10 times in breakdown field strength as large as that of silicon, a voltage nearly 10 times as high as that of a silicon device is applied to the SiC semiconductor device in use. For that reason, an electric field of the strength ten times as large as that of the silicon device is applied to a gate insulating film formed within the trench. As a result, the gate insulating film may be easily broken at the corners of the trench.
In order to solve the above problem, patent literature 1 proposes a structure in which a p-type impurity is ion-implanted to a part lower than a bottom (bottom surface) of the trench configuring the trench gate structure to form a p-type layer. With the formation of the above p-type layer, an electric field concentration at the bottom of the trench can be reduced, thereby being capable of restricting the gate insulating film from being broken.
Also, when the corners of the trench configuring the trench gate structure are angular, the electric field concentration is more generated, leading to a reduction in the lifetime of the gate insulating film. For that reason, patent literature 2 proposes to perform a rounding process for rounding the corners of the trench by conducting hydrogen etching. Because the corners of the trench can be thus rounded to inhibit the gate insulting film from being partially thinned, the breakage of the gate insulating film at the time of turning on and off the MOSFET can be restricted.